Optical beamsplitters are used in a number of different applications. In spectroscopy and other instrumentation systems, for example, a beamsplitter is used to direct excitation energy of one wavelength toward a sample along a first optical path and to direct emitted light that has been excited from the sample to sensing components along a second optical path, which can be in the opposite direction from some portion of the first optical path. The beamsplitter reflects light of at least one wavelength band and transmits light of another wavelength band or bands.
The schematic diagram of FIG. 1 shows one exemplary optical apparatus that uses a beamsplitter. In a fluorescence microscope 10, excitation light of wavelength λ1, directed through a lens 34 from a light source 12, is first filtered by an excitation filter 20 and then reflected from a dichroic beamsplitter 22 before being directed to a sample 14 through an objective lens 24. Fluorescent molecules in sample 14 absorb this light, and then emit longer-wavelength fluorescence light of wavelength λ2, some of which is captured by the objective lens 24 and imaged through one or more secondary lenses 26 onto a detector 28 (such as a human eye or charge-coupled device or CCD camera). The fluorescence is transmitted through the same dichroic beamsplitter 22 as well as through an emission filter 32 which is required to block all unwanted background light as well as light from the excitation light source 12.
A similar configuration to that shown in FIG. 1 is generally used for other types of spectroscopy measurement systems in which a generally shorter-wavelength excitation light source generates light at longer wavelengths to be spectrally detected, such as Raman spectroscopy. For reasons of mechanical convenience, compactness, and stability, it is generally desirable for the excitation light to be incident on the imaging path at a 90° angle, such as shown in FIG. 1. This light is incident on the dichroic beamsplitter at 45°.
In many systems a plate dichroic beamsplitter is used, which comprises a multilayer thin-film coating applied to one surface of a thin parallel plate of glass. The back surface of this filter may be anti-reflection coated to minimize loss of light and extraneous reflections.
In practice, a plate dichroic beamsplitter is a workable solution only when the light between the objective lens 24 and the secondary lens 26 is highly collimated, as in the FIG. 1 example. If this light is not collimated, however, the tilted parallel plate of glass on which the thin-film coating is formed causes increased spherical aberration as well as appreciable asymmetric aberrations, such as coma and astigmatism, at the image. Furthermore, the tilted plate causes a slight lateral shift of the optical axis OA. In some systems it is not desirable or possible to tolerate a lateral shift of the beam of image-bearing light, and therefore, of the optical axis, as caused by the tilted parallel plate.
To avoid these aberrations in systems where the light is not collimated, as well as to minimize or eliminate the lateral beam shift, some systems use cube or cubic dichroic beamsplitters, as shown in FIG. 2. A cube dichroic beamsplitter 120 is formed as a type of composite prism from two right-angle component prism elements 122a and 122b, each joined to the other along the facing surface that is its hypotenuse, with a multilayer thin-film coating 124 applied to one hypotenuse or the other hypotenuse. This arrangement embeds dichroic coating 124 within the glass substrate. Optical contact between the component prism elements can be effected in a number of ways familiar to those skilled in the optical arts, such as using an index-matching cement, or employing what is known as direct optical contact (glass-to-glass bonding by weak van der Waal's forces), or with strong, chemically activated molecular glass-to-glass bonding. In some cases, there can also be a fixed gap such as an air gap maintained between the two component prisms. Because light only enters and exits any glass surface at or near normal incidence (0°) with these prisms, the cube beamsplitter approach solves the optical aberration and beam shift problems described earlier.
One notable drawback of the cube approach, however, relates to high angles of light incidence on the embedded multilayer thin-film coating 124 that lies within the prism. It is well-known that filter response for thin film filters changes with angle, so that multilayer thin film coatings tend to degrade in performance as the angle of incidence increases.
With the plate beamsplitter, as in FIG. 1, light is incident on the dichroic surface at 45° in air. However, due to Snell's law of refraction, the light bends upon entering the glass substrate, so that its incident angle, relative to the thin film layers coated on the glass, is refracted to about 28° (assuming an index of refraction near 1.5 for the glass substrate of beamsplitter 22). This is a suitable angle for reasonable dichroic coatings performance and the plate beamsplitter 22 can provide effective separation of light for many applications with incident light in this range.
However, the case is different with the cube beamsplitter. Light traveling within the cube substrate does not refract as it nears the multilayer thin-film coating and is incident on the thin-film coating at a much higher angle of incidence than it is in the case of the plate beamsplitter. It is much more difficult to design and fabricate a dichroic beamsplitter coating with a steep, well-defined edge transition between reflection and transmission for light incident at 45°. At higher incidence angles, polarization differences compromise beamsplitter performance. P-polarized light experiences much lower reflection than s-polarized light, and the wavelength location of a filter edge tends to be very different for s- and p-polarized light. This behavior, termed “polarization splitting”, tends to broaden transition edges of the filters. As a result, the spectral performance of the cube dichroic beamsplitter that has an embedded coating can be disappointing, resulting in poorer overall system efficiency and, in many cases, resulting in lower signal sensitivity.
Conventional cube beamsplitter designs that use embedded multilayer thin-film coatings are hampered by poor performance at high incidence angles and are unable to benefit from the advantages dichroic beamsplitter thin-film coatings have at lower angles of incidence. For example, fewer thin film layers are needed for a given amount of reflectivity or edge steepness at lower angles of incidence. This has advantages of reduced cost and improved edge steepness over multilayer coatings designed for higher incidence angles. As yet another consideration, a coating with fewer layers generally also exhibits lower group delay dispersion (all other performance parameters being equal), with significantly improved performance for beamsplitters that reflect femtosecond laser pulses.
Still another consideration relates to the demands of the optical system itself. Various types of composite prisms with embedded dichroic coatings or coatings applied to one or more surfaces have been designed for color splitting or combining, such as in camera and projection apparatus, for example. None of these conventional solutions, however, is well-suited for use in a spectroscopy measurement system. In the spectroscopy apparatus, input light at one wavelength is reflected toward a sample along an optical imaging path, while light of a different, typically longer wavelength is transmitted through the beamsplitter along the same optical path in the opposite direction, entering and exiting the glass block at or near normal incidence.
In summary, although the cube beamsplitter has clear advantages that relate to mounting, light handling, and durability, the poor relative spectral performance of these devices makes them less desirable than plate beamsplitters for light separation in many applications.
Therefore, there is a need for a dichroic beamsplitter cube that supports orthogonal input and output light paths, that takes advantage of the low aberration and beam shift of a glass cube or prism when contrasted with a plate dichroic in imperfectly collimated light, and that has improved spectral performance over conventional cube beamsplitter designs.